This relates to solid-state image sensors and, more specifically, to image sensors with small size pixels that use photoelectric films for conversion of light into electrons or holes. In such sensors, it can be difficult to achieve a charge transfer scheme for transferring charge from the charge integration node to the charge detection node that does not introduce significant kTC noise into the signal
Conventional image sensors sense light by converting impinging photons into electrons or holes that are integrated (collected) in sensor pixels. Upon completion of each integration cycle, the collected charge is converted into voltage signals, which are then supplied to corresponding output terminals associated with the image sensor. Typically, the charge-to-voltage conversion is performed directly within the pixels, and the resulting analog pixel voltage signals are transferred to the output terminals through various pixel addressing and scanning schemes. The analog signal can sometimes be converted on-chip to a digital equivalent before being conveyed off-chip. Each pixel includes a buffer amplifier, commonly a source follower (SF), which is used to drive output sensing lines that are connected to the pixels via respective address transistors.
After charge-to-voltage conversion is complete and after the resulting signals are transferred out from the pixels, the pixels are reset before a subsequent integration cycle begins. In pixels having floating diffusions (FD) serving as the charge detection node, this reset operation is accomplished by momentarily turning on a reset transistor that connects the FD node to a fixed voltage reference for draining (or removing) any charge transferred on the FD node. However, removing charge from the floating diffusion node using the reset transistor generates kTC-reset noise, as is well known in the art.
In conventional CMOS image sensors, the light-to-charge conversion occurs in the silicon substrate in a separate pixel area that is dedicated for the purpose of converting light into charge. This could be, for example, a simple n+ p diode or a pinned photodiode. However, another class of CMOS image sensors exists in which the CMOS sensor circuitry built in the substrate is overlaid by an organic photoelectric conversion layer (sometimes referred to as a charge generating layer). In this type of image sensor, the charge generating layer covers essentially the whole pixel area. Thus, the area in the substrate that was previously used exclusively for light-to-charge conversion circuitry may not be needed.
This type of image sensor may eliminate the need for micro-lenses over the sensor pixels. This type of image sensor may also be configured to convert light into charge with high quantum efficiency (QE) and does not necessarily need to rely on the intrinsic silicon properties. It is thus possible to reduce the thickness of the overlaid material layer, which will in turn reduce the pixel cross talk and the demands on the sensor camera optics. An example of this type of sensor has been described in a paper entitled: “CMOS Image Sensor with an Overlaid Organic Photoelectric Conversion Layer: Optical Advantages of Capturing Slanting Rays of Light,” by Mikio Ihama et al., incorporated herein as a reference.
FIG. 1 is a simplified cross-sectional side view of a conventional image sensor pixel 100. As shown in FIG. 1, conventional image sensor pixel 100 includes silicon p type doped epitaxial layer 101 deposited on a p+ type doped substrate 102. Oxide layer 112 is formed on top of epitaxial layer 101 and also extends and fills the shallow trench isolation (STI) regions 103. Oxide layer 112 also serves as a gate oxide for the pixel transistors. A reset transistor is formed by n+ diffusion region 104, n+ floating diffusion (FD) region 105, and gate poly-silicon region 111. Similarly, the source follower (SF) transistor is formed by n+ region 106, n+ region 107, and gate region 110. A pixel row select transistor is formed by n+ region 107, gate region 109, and n+ source region 108.
Oxide isolation layer 113 is deposited over the poly-silicon gates to provide isolation for the pixel metal wiring, which is only partially shown as connection 117 in pixel 100. Wire 117 provides the necessary electrical connection of the FD to the gate of the SF transistor. Metal vias 115, 116, 124, and 125 provide connections between the circuit elements that are built within the silicon substrate and on top of it to the corresponding pixel wiring. Additional oxide isolation layers such as oxide isolation layer 114 are deposited on the metal interconnects wiring. For simplicity, not all of these oxide isolation layers are shown in FIG. 1.
Layer 118 serves as a light shield to protect the substrate circuits from the impinging light. Metal layer 119, which is electrically connected to the FD, is the anode electrode for the organic photoelectric layer 121. Transparent and conductive layer 123 serves as a cathode electrode for the organic photoelectric layer. Cathode electrode 123 is biased by a negative bias Vb, approximately equal to −10V. Pixel 100 also includes electron blocking layer 122 and hole blocking layer 120. These layers are necessary to prevent the direct injection of charge from the electrodes into the structure and thus to make sure that only the photo-generated carriers are detected. Photons 126 enter pixel 100 from front side 100F.
During operation of pixel 100, the FD charge detection node is reset by momentarily pulsing gate 111 positive, and the photo-generated electrons are integrated on the FD node. The photo-generated holes flow directly into cathode electrode 123. The potential change of the FD node is sensed by the source follower transistor, and this potential change is transferred onto the column readout circuits when the row select transistor is momentarily pulsed on. When the sensing is completed, the FD node is reset again. It is common practice to use a double sampling (DS) signal processing technique to detect only the difference between the signal level before and after the reset. This eliminates any transistor threshold variation from the signal and ensures sensing uniformity across the array. However, this procedure has been known to generate kTC-reset noise, as mentioned above. Moreover, pixel 100 cannot be used in a global shutter (GS) mode of operation where all the pixels are exposed to the impinging light simultaneously without any time skew.
It may therefore be desirable to be able to provide improved image sensors with low-noise charge reset and charge integration schemes.